This invention relates to spectroscopy, and more particularly to ion cyclotron resonance spectroscopy.
Ion cyclotron resonance is well known, and provides a sensitive and versatile means for detecting gaseous ions. In this regard, it is well known that a moving gaseous ion in the presence of a static magnetic field is constrained to move in a circular orbit in a plane perpendicular to the direction of the magnetic field, and is unrestrained in its motion in directions parallel to the magnetic field. The frequency of this circular motion is directly dependent upon the strength of the magnetic field and the charge-to-mass ratio of the ion. When such orbiting ions are subjected to an oscillating electric field disposed at right angles to the magnetic field, those ions having a cyclotron orbital frequency equal to the frequency of the oscillating electric field absorb energy from the electric field and are accelerated to larger orbital radii and higher kinetic energy levels. Because only the resonant ions absorb energy from the electric field, they are distinguishable from non-resonant ions upon which the field has substantially no effect.
Various methods of and apparatus for taking advantage of the foregoing phenomena and utilizing it to measure the number of ions having a particular resonant frequency have been proposed and are in use. These devices are generally referred to as ion cyclotron resonance mass spectrometers.
In the omegatron type of ion cyclotron resonance mass spectrometer, gaseous ions are generated by bombardment of a gaseous sample with moving electrons. These ions are then subjected to mutually perpendicular magnetic and oscillating electric fields. The electric fields accelerate the resonant ions to higher velocities and larger oribtal radii. Ultimately these ions accelerate to a point where they impinge upon a collector plate. The resulting ion current is measured and recorded.
In another type of ion cyclotron mass spectrometer, ions having a resonant frequency equal to the frequency of the oscillating electric field are accelerated and the resultant power absorbed from the electric field is measured. The measured power is related only to the resonant ions, and not to ions having other resonant frequencies. Thus, detection of the absorbed power results in a measurement of the number of resonant gaseous ions of a particular mass-to-charge ratio present in a sample. Obviously, an ion mass-to-charge ratio spectrum of a particular ionized gas sample is obtained by scanning and detecting. Scanning can be accomplished by varying the frequency of the oscillating electric field, the strength of the applied magnetic field, or both, so as to bring ions of differing mass-to-charge ratios into resonance with the oscillating electric field. An example of an ion cyclotron resonance mass spectrometer utilizing such a power absorption detection technique is described in U.S. Pat. No. 3,390,265 entitled "Ion Cyclotron Resonance Mass Spectrometer Means for Detecting the Energy Absorbed by Resonance Ions" issued to Peter M. Llewellyn on June 25, 1968.
Other U.S. patents disclosing various related ion cyclotron resonance mass spectrometers methods and apparatus, and improvement thereto, are: U.S. Pat. No. 3,446,957 entitled "Ion Cyclotron Resonance Spectrometer Employing Means for Recording Ionization Potentials" issued to David E. Gielow et al on May 27, 1969; U.S. Pat. No. 3,475,605 entitled "Ion Cyclotron Double Resonance Spectrometer Employing a Series Connection of the Irradiating and Observing RF Sources to the Cell" issued to Peter M. Llewellyn on Oct. 28, 1969; U.S. Pat. No. 3,502,867 entitled "Method and Apparatus for Measuring Ion Interrelationships by Double Resonance Mass Spectroscopy" issued to J. L. Beauchamp on Mar. 24, 1970; U.S. Pat. No. 3,505,516 entitled "Ion Cyclotron Resonance Spectrometer Employing an Optically Transparent Ion Collecting Electrode" by D. E. Gielow et al issued Apr. 7, 1970; U.S. Pat. No. 3,505,517 entitled "Ion Cyclotron Resonance Mass Spectrometer with Means for Irradiating the Sample with Optical Radiation" issued to P. M. Llewellyn on Apr. 7, 1970; U.S. Pat. No. 3,511,986 entitled "Ion Cyclotron Double Resonance Spectrometer Employing Resonance in the Ion Source and Analyzer" issued to P. M. Llewellyn on May 12, 1970; U.S. Pat. No. 3,535,512 entitled "Double Resonance Ion Cyclotron Mass Spectrometer for Studying Ion-Molecule Reactions" issued to J. D. Baldeschwieler on Oct. 20, 1970; and U.S. Pat. No. 3,677,642 entitled "Ion Cyclotron Resonance Stimulated Low-Discharge Method and Apparatus for Spectral Analysis" issued to J. D. Baldeschwieler on July 18, 1972.
In general, all of the foregoing patents disclose ion cyclotron resonance mass spectrometers wherein adverse space charge effects are reduced by continuously ionizing a gas sample within a first region of a sample chamber, and subjecting the ions thus produced to transverse magnetic and static electric fields. These fields move the ions along cycloidal paths in a direction perpendicular to both fields to a second region of the same sample chamber removed in space from the first region. In the second region, the ions are subjected to the combined influence of the magnetic field and an oscillating electric field lying at right angles thereto. In accordance with general ion cyclotron resonance phenomena discussed above, the ions having a resonant frequency equal to the frequency of the oscillating electric field absorb energy from that field and the energy absorption is detected to provide a measure of the number of such resonant ions. Because the resonant ions are detected in a second analyzing region, which is spatially distinct from the first ionizing region, the effect of space charge in the analysis is reduced.
A U.S. patent disclosing a somewhat different type of ion cyclotron resonance mass spectrometer is U.S. Pat. No. 3,742,212 entitled "Method and Apparatus for Pulsed Ion Cyclotron Resonance Spectroscopy" issued to Robert T. McIver, Jr. on June 26, 1973. The spectrometer disclosed in this patent includes a single-section ion cyclotron resonance cell. In this cell, ions are formed during a known first time period, allowed to react with neutral molecules for a second time period, and detected in a third time period. The detection of ions of a particular mass-to-charge ratio is achieved by suddenly changing the resonant frequency of the desired mass-to-charge ratio ions so as to equate their resonant frequency to the fixed frequency of a marginal oscillator detector. (Except during the "detect" time period, the ion cyclotron frequencies are not equal to the marginal oscillator frequency.) The marginal oscillator frequency then provides an output signal proportional to the number of ions that absorb energy from it at a given instant of time. The required sudden change in the cyclotron frequency of the ions of a given mass-to-charge ratio is achieved either by a sudden change in the value of the applied magnetic field or by a sudden change in the magnitude of the static electric field which is used to "trap" the ions in the ion cyclotron resonance cell. An alternative means for initiating the ion cyclotron resonance detection period is to suddenly change the amplitude of the radio frequency level of the marginal oscillator from zero volts to some higher level. After the ion cyclotron resonance detection period is completed, a "quench" electric field pulse is applied to remove all ions from the ion cyclotron resonance cell. The total operational sequence (ion formation, delay period for ionmolecular reactions, ion cyclotron resonance detection, ion removal) is then repeated.
One of the major disadvantages of all of the above-noted prior art ion cyclotron mass resonance spectroscopy methods and apparatus is that ion cyclotron resonance detection is limited to a single frequency (and therefore a single mass-to-charge ratio) at any instant in time. In order to obtain a wide-range mass-to-charge ratio spectrum of a given ionized gaseous sample, it is necessary to vary either the magnetic field (or the frequency of the oscillating electric field, or both) so as to equate the resonance of the various ions with the resonance of the oscillating electric field. In this regard, by way of example, using a fixed oscillator detector frequency of 153 KHz, it requires about 25 minutes to obtain a typical mass-to-charge ratio spectrum by varying the applied magnetic field by an amount adequate to cover a mass range of 15 atomic mass units to 240 atomic mass units, for singly charged ions.
Therefore, it is an object of this invention to provide a new and improved method of and apparatus for ion cyclotron resonance spectroscopy that provides a wide-range mass-to-charge ratio spectrum for a given ionized sample in a relatively short period of time.
In addition to the slow spectrum processing time of prior art ion cyclotron methods and apparatus, other disadvantages exist. For example, the resolution of the resultant signals is relatively fixed and cannot easily be varied to improve the accuracy of the resultant information. More specifically, prior art methods and apparatus are not readily adapted to vary the resultant signal-to-noise ratio in order to improve resolution. It will be appreciated that the ability to trade one of these factors against the other is of particular importance when the sample being analyzed is very dilute.
Therefore, it is also an object of this invention to provide a new and improved method of and apparatus for ion cyclotron spectroscopy wherein resolution and signal-to-noise ratio can be readily varied.
The utilization of Fourier transform techniques in infra-red and nuclear magnetic resonance spectroscopies has been proposed by the prior art. In general, Fourier transform techniques provide for the detection of a complete speectrum of information in the time normally required to scan through a single frequency-resolution element using conventional scanning techniques. In this regard, U.S. Pat. No. 3,475,680 entitled "Impulse Resonance Spectrometer Including a Time Averaging Computer and Fourier Analyzer" issued to W. A. Anderson et al on Oct. 28, 1969 suggests the use of Fourier techniques in a variety of spectroscopies; however, not ion cyclotron resonance spectroscopy. Further, U.S. Pat. No. 3,530,371 entitled "Internal Field-Frequency Control for Impulse Gyromagnetic Resonance Spectrometers" issued to F. A. Nelson et al on Sept. 22, 1970, suggests a Fourier method for the specialized purpose of controlling the magnetic field intensity in various spectrometers, including ion cyclotron resonance spectrometers. Finally, U.S. Pat. No. 3,461,381 entitled "Phase Sensitive Analog Fourier Analyzer Read-Out for Stored Impulse Resonance Spectral Data" issued to F. A. Nelson et al on Aug. 12, 1969 suggests an analog techique for obtaining the Fourier transform of a nuclear magnetic free-induction response to a pulse magnetic field excitation. This technique is suggested for application to ion cyclotron resonance spectrometers. While this and other similar prior art does broadly suggest the application of Fourier techniques to ion cyclotron spectroscopy, they do not disclose a method of or apparatus for utilizing Fourier transform techniques to provide a Fourier transform ion cyclotron resonance spectrometer. In fact, these patents merely disclose impulse excitation of the transient spectral response, which excitation is not particularly suitable for utilization in a Fourier transform ion cyclotron resonance spectrometer for reasons which are set forth below.
Therefore, it is a general object of this invention to provide a new and improved method of and apparatus for ion cyclotron resonance spectroscopy.
It is a further object of this invention to provide a Fourier transform ion cyclotron resonance spectrometer.
It is yet another object of this invention to provide a method of ion cyclotron resonance spectroscopy that utilizes Fourier transform techniques to obtain a spectrum of ion cyclotron resonance information.
It is yet another object of this invention to provide a Fourier transform ion cyclotron resonance spectrometer suitable for obtaining an ion cyclotron resonance spectrum of a specified mass-to-charge range and resolution in a time period much shorter than that required by prior art ion cyclotron resonance spectrometers.
It is a subsidiary object of this invention to provide an ion cyclotron resonance spectrometer that employs a fixed-field magnet for generating the necessary magnetic field.
It is yet another object of this invention to provide an apparatus for exciting ions of many different mass-to-charge ratios in a short period time and rapidly detecting the excited ion cyclotron motion of such ions.
It is a further object of this invention to provide a method of and an apparatus for ion cyclotron resonance spectroscopy that provides a technique for reducing the effect of erroneous information created by noise and the like.